1. Field of the Invention
The present invention relates to a variable angle wobble plate type compressor with a unit for changing compressor displacement. More particularly, the present invention relates to a variable angle wobble plate type compressor including a suction chamber, a discharge chamber, and a crankcase, wherein a piston stroke is varied through a change in an inclination of the wobble plate from a fully inclined position to a substantially non-inclined position nearly vertical to a drive shaft of the compressor, which change is caused by a pressure difference between a suction pressure and a crankcase pressure, whereby the compression capacity of the compressor is changed over a wide range.
2. Description of the Related Art
A typical conventional variable displacement compressor applicable for an air conditioning system of a vehicle is disclosed in U.S. Pat. No. 4,428,718 to Skinner. This compressor is provided with a bellows in a suction chamber to detect suction pressure. When the suction pressure decreases below a predetermined value due to a decrease in a cooling load or due to a high-speed rotation of the compressor, the bellows is expanded according to the variation of balance between the suction pressure and atmospheric pressure to actuate a valve mechanism. A communication passage between the crankcase and suction chamber is then closed and a communication passage between a discharge chamber and the crankcase is opened, to increase the crankcase pressure so that the pressure difference between the crankcase pressure and suction pressure will be increased. As a result, a pistol stroke is reduced to reduce the inclination of a wobble plate which causes the piston to reciprocate, so that the suction pressure will be prevented from falling below a predetermined value, and the compressor displacement will be reduced.
However, when the suction pressure is temporarily decreased due to, for instance, a sudden acceleration, the bellows of the conventional variable displacement compressor mentioned above rapidly detects the change of suction pressure and actuates the valve mechanism, causing a high-pressure discharge gas to be sent into the crankcase and excessively increasing the crankcase pressure; although, in such a sudden acceleration, the piston stroke is automatically reduced according to the decrease of suction pressure, to start a small displacement operation without the need to increase the crankcase pressure. Due to the above, even if the revolution speed is decreased after the sudden acceleration operation, the pressure difference between the suction chamber and crankcase is small due to the reduction of the number of revolutions and the pressure in the suction chamber is increased by the insufficient displacement in the small displacement operation, so that the excessively heightened pressure in the crankcase can be reduced only gradually and the small displacement operation will be continued with the reduced piston stroke. As a result, the temperature in a vehicle cabin rises. To lower the temperature to an optimum value, the inclination of the wobble plate must be once returned to the maximum inclination, i.e., a delay occurs before the optimum temperature can be reached. Further, the bearing pressure at the shaft seal is increased because the crankcase pressure is excessively increased for every sudden acceleration, causing a problem in that the durability of the shaft seal mechanism is lowered.
In order to eliminate the above-mentioned problems of the conventional variable displacement wobble plate type compressor, U.S. patent application Ser. No. 839,908 to be assigned io the same assignee as the present patent application discloses a novel control means for controlling a compressor displacement of a variable displacement wobble plate type compressor, in which the pressure in the crankcase interior or chamber is kept substantially at a predetermined constant value during ordinary operation of the compressor, and the piston stroke of the compressor is controlled according to a pressure difference between the pressure in the crankcase interior, which is kept substantially at the constant value, and the suction pressure which varies according to a change in the cooling load, etc. The inventors of the present application have continued an investigation of the performance of a wobble angle controlling unit of a variable displacement wobble plate type compressor on the basis of the invention disclosed in the above-mentioned related application. That is, the present inventors have endeavored to develop a compressor of the type mentioned above, in which a return of the wobble plate from a fully inclined position thereof to the least inclined position thereof is smoothly carried out.